A former Marine charged with the gory stabbing deaths of six people in California told investigators he targeted homeless people in part because they were vulnerable, and that he believes he has a 'killer gene'.

Itzcoatl Ocampo, a 23-year-old Iraq War veteran, was initially arrested for the stabbing deaths of four homeless men earlier this year.

Ocampo later confessed to two more murders - the October killings of his high school friend Eder Herrera's mother and brother in Yorba Linda.

The revelations come as Ocampo faces six felony counts of murder for allegedly stalking and stabbing his victims in killings authorities said were carried out with a roughly seven-inch military-style blade and that terrorized Orange County's homeless community.

Ocampo has pleaded not guilty to the charges.

If convicted, Ocampo could face the death penalty. The district attorney has not yet decided whether to seek capital punishment in the case.

Ocampo told investigators he started to kill homeless men because they were 'available and vulnerable' and continued stabbing his victims even after believing they were dead, according to the transcript.

Ocampo said he looked at Penthouse magazine before the attacks 'to pump himself up,' Wyatt said.

'He seemed to get excited when he was talking about the actual kill,' Wyatt testified. 'So, I asked him if he was aroused by the act of killing.'

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The victims: (L-R) James
McGillivray was stabbed outside a shopping centre on December 20; Lloyd
Middaugh was found stabbed on a riverbed on December 28; and Paulus
Cornelius Smit was found stabbed outside a library on December 30

Service: A relative holds a photograph of Itzcoatl posing proudly in his Marines uniform. He was said to have become paranoid and delusional after his deployment

Worried: Refugio Ocampo, the father of the 23-year-old who is the prime suspect in the killings, is himself homeless and said the Iraq war killed the man his son used to be

Ocampo at first questioned the word arousal, according to the transcript, but then added, 'my balls felt like they were going to explode, and I knew that I had the killer gene.'

Ocampo was arrested in January after a witness helped chase down a suspect following the stabbing of a fourth homeless victim outside a fast-foot restaurant in Anaheim, about 26 miles southeast of Los Angeles. Ocampo was initially charged with the four murders and prosecutors last month added charges for the Yorba Linda killings.

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Detectives said Wednesday they saw similarities in the cases of the Yorba Linda deaths and the stabbings of four homeless men in December and January.

Detectives focused on the severity of the attacks, the number and the type of wounds, and the proximity of Ocampo's home to the Yorba Linda killings, the Orange County Register reported.

Murder weapon: Orange County District Attorney Tony Rackauckas holds up picture of a knife similar to the one he accused of being used by Itzcoatl Ocampo in the killings

Normal young man: Itzcoatl Ocampo's family has released these images of the young man, showing him in happier times

Authorities have linked Ocampo to a killing spree targeting four homeless men: James Patrick McGillivray, 53, who was stabbed December 20 near a shopping center in Placentia; Lloyd Middaugh, 42, who was found December 28 near a riverbed trail in Anaheim; Paulus Smit, 57, who was stabbed to death outside a Yorba Linda library on December 30; and John Berry, 64, who was stabbed to death on the day Ocampo was arrested.

Each of the four men was stabbed more than 40 times with a weapon believed to be a 7-inch fixed-blade military-type knife, authorities said.

Locations: Police in Orange County, California say Ocampo was responsible for four killings since December 2011

Before Ocampo's January 13 arrest, police had fanned out across the county better known as the home to Disneyland and multi-million dollar beachfront homes to urge the homeless to be careful and seek shelter indoors.